Magnetron modulation circuit



IWW, E95@ c. I. STEIGERWALT 29540,?64

MAGNETRON II/IODULATION CIRCUIT Filed Dec. 10, 1945 Patented Feb. 6, 19,51

2,540,7 64 MAGNETRON MODULATION CIRCUIT Oliver I. Steigerwalt, Eri

United States of Secretary of War e, Pa., assignor to the America as represented by the Application December 10, 1945, Serial No. 634,100

3 Claims.

This invention relates to electrical apparatus and more particularly to the modulation of magnetron radio frequency oscillators.

An important object of the invention is to provide means for modulating a magnetron radio frequency oscillator with modulation frequencies in the range from low audio frequencies to upwards of 5 mc. p. s.

A further object of the invention is to prevent the magnetron oscillator from dropping out of oscillation when the modulation is applied.

Other objects and advantages of the invention will be apparent during the course of the following description.

The present invention is essentially a currentmodulating device. A tetrode or pentode modulator tube is placed in series with the magnetron plate circuit. The modulation voltages are applied to the control grid of the modulator tube and vary the current through the modulator tube and magnetron.

In the accompanying drawing forming a part of this specification and in which like numerals are employed to designate like parts throughout the same: l'

Fig. 1 isI a simplified schematic circuit diagram illustrating one embodiment of the invention; and

Fig. 2 is a typical plate current versus plate voltage characteristic for a magnetron radio frequency oscillator.

In Fig. l, wherein is shown a preferred embodi ment of the present invention, the numeral IU designates a split-anode radio frequency magnetron oscillator having a central directly-heated cathode II surrounded by a concentric splitanode of two semicylindrical sections I2 and I3. Cathode lI is supplied by filament transformer It.

A variable short-circuited parallel wire radio frequency transmission line I5 is connected across anodes I2 and I3. The radio frequency output transmission line i6 is taken off between a movable short-circuiting bar I'I and an adjustable tap I 8.

The D. C; polarizing voltage between cathode l I and anodes I2, I3 is obtained from power supply 26. In series with power supply 20 and magnetron Il) is a current-modulation tetrode vacuum tube 2 I.

Modulator tube 2i may be a pentode or a tetrode, its requisite being that it have a high plate resistance. The modulation voltage 22 and appropriate fixed grid bias 23 are applied between grid and cathode of modulator tube 2l.

comprising two chokes :made clear by reference to Fig. 2.

l started (i. e. voltage In series with power supply 20 and the remainder of the circuit is a low pass filter comprising series chokes 24 and 25 and shunting condenser 26.

The operational features of the present invention are as follows:

Split-anode magnetron I 0, as is well known, acts as a negative resistance oscillator. The variable short-circuited transmission line I5 serves as the resonant tuned-circuit for magnetron I El. The frequency ofl oscillation may be varied by adjusting the length of the line I5 by means of movable short-circuiting bar I1. Short-circuiting bar Il is both the R. F. and the D. C. ground.

The radio frequency output is taken across the shorting bar Il and the movable tap I8 on transmission line I5. By means of this movable tap I8 an impedance match may be obtained between the output circuit and the magnetron oscillator IIJ.

The modulator tube 2| is placed in series with the power supply 2o and magnetron ID and functions as a current modulating device. The quiescent value of the current will be determined by the fixed grid bias 23. Variation'of the grid bias by the modulation voltage 22 will vary the current through magnetron I8 as a function of the modulating voltage 22. The low pass filter 24, 25 and the condenser 25 keeps the modulation frequencies out of the power supply 20.

The particular advantage of current modulation for the magnetron type of oscillator will be Fig. 2 is a plate current versus plate voltage characteristic curve for a typical magnetron oscillator. As the cathode-anode voltage is increased (with a constant magnetic field), the characteristic curve proceeds along the path 3I-32-33, with very little current being drawn. The tube does not oscillate until a critical voltage 33 is applied. Voltage 33 may be considerably in excess of the voltage drop across the tube once oscillation has 34). If voltage 33 tends to be maintained (because of good regulation of the power supply) the current through the tube will become excessive. Conversely, if the anode voltage is reduced, the magnetron will drop out ofl oscillation, the characteristic curve proceeding along the line 34- 32-'3I.

It should be evident then that variation of the anode voltage (which has been the customary method for simple on-off modulation) is unsatis factory for magnetron modulation in general. A

3 series current-modulating device is particularly advantageous. A modulator which is capable of limiting the current through the magnetron as well as modulating the current is embodied in the present invention. The use of the high plate resistance tetrode or pentode modulator tubefl accomplishes the desired current modulation.

The quiescent current is controlled by the fixed.

grid bias source 23 and modulation of the current is eiiected by the modulation voltage 22. The instantaneous sum of grid bias 23 and modula tion voltage 22 constitutes the cathode-grid. voltage of modulator tube 2|.

rihe D. C. ground connection is placed; atthe cathode of modulator tube 2 ii. This necessitates the use of an ungrounded power supply 29 but re- Y Vment of the present invention, it will be obvious to those skilled in the art that variousV changes and modications may be made therein without departing from the scope of the invention.,

What is claimed is:

1i A modulated radio frequency oscillator including. a split-anode negative resistance magnetron, said magnetron having a central'. filament surrounded by a concentric anode of two semicylindrical sections, a resonant radio vfrequency transmission line connected across the anode sections of said magnetron and serving as the resonant circuit of said magnetron, power supply means for maintaining a potential difference between said iilament and said split-anode of said magnetron, a current-modulation having a control grid, said tube being connected in series with said magnetron and said power supp-ly: means, a low-pass filter connected in series with said power supply means, said tube and said magnetron, said magnetron having only its lament and anode connected to said tube' and 4power supply means, said filter serving to minimize the radio frequency power coupled to said power supply means and means for applying modulating voltages to said control grid whereby said voltages applied to said control grid will vary the resistance of said current-modulation tube thereby modulating the current through said Vacuum tubeV 4 magnetron and the radio frequency output thereof.

2. A modulated radio frequency oscillator including a magnetron having a cathode and a plurality of anode sections, a power supply for impressing a potential difference between said cathode and said anode sections, a currentmodulating vacuum tube having a control grid, saidtube being connectedA in series with said power supply and said magnetron, a low-pass filter connected to said power supply and adapted to minimize the radio frequency power coupled to said power supply, said magnetron having only its cathode-1 and'. anodefsections connected to said power supply and meansfor applying modulating voltages to said control grid whereby the resistance of said current-modulating tube maybe varied thereby varying the current through said magnetron and the radio frequency output thereof.

3; A modulated; radio frequency oscillator including a split-anode negative resistance magnetron, said magnetron having a central'iilament surrounded by two concentric semi-cylindrical anode; sections, a resonant radio frequency transmission line connected across the anode section of said: magnetron and constitutingv the resonant circuit of said magnetron, powerr supply means for maintaining a potential difference between said mamen-t and the anode sections of said magnetron, a current modulation vacuum tube having a control grid, said tube having a cathode; connected. to said transmission line, a connection from said cathode and said transmission line to ground, said tube having an anode connected to oneterminal of said power supply means,` the filament of said magnetron being connected to the other terminal of said power supplymeans, and means for applying modulation voltages: toV said control grid, whereby said voltages applied to said control grid vary the resistance of the cur-v rent modulation tube thereby modulating. the current throughV said magnetron and the radio ireduencyL output thereof.

OLIVER I. STEGERWALT.

REFERENCES CITED The following references are of record in the i-lle of this patent:

UNITED STATES PATENTS 

